There exists a number of optical systems that require the high optical power densities which are provided by two-dimensional diode laser arrays. For example, high power densities are needed for the optical pumping of Nd:YAG lasers and the like. One structure that can supply such power is a stacked array assembly, such as those described in U.S. Pat. No. 4,716,568 to Scifres et al. Stacked array assemblies include linear diode laser arrays or "laser bars," typically with aperture lengths near or in excess of 1 cm, individually bonded to submounts. The submounts are in turn stacked one above the other in such a manner that the laser apertures point in a common direction, thereby creating a two-dimensional structure with high power density. Very dense structures are capable of providing pulsed outputs in excess of 3 kw/cm.sup.2, while less dense structures are capable of providing continuous outputs in excess of 100 W/cm.sup.2 Many of such array assemblies can be butted together in both dimensions to produce very large area outputs.
A key element of the array assemblies is the structure of the submounts on which the laser bars are mounted and which must be bonded together to complete the assembly. The submount must provide adequate thermal conductivity to remove waste heat from the laser bar. It must conduct electricity to the laser bar, as well as provide electrical interconnection between adjacent submount units. The submounts must also have sufficient mechanical integrity and be bonded together in a manner that minimizes the transmission of life-limiting mechanical stress to the laser. For example, in order to minimize stress on the lasers, the submount units in the aforementioned Scifres et al. patent may be a combination of support plates, into which the laser bars are mounted, together with separate spacer plates for providing a gap, where the submount units are stacked together, between the laser bars with their wire bonds and the support plate of the adjacent submount unit. Alternatively, the submount units may be unitary members shaped to provide both support for the laser bars and the necessary gap or spacing between the laser bar and the adjacent submount. The assembly of stacked submount units is held together in the Scifres et al. patent by bolts passing through holes in the submount units.
Another important criterion that should be met in a stacked array assembly is accurate registration of the submount units over long distances. One reason that registration is important is that it is desirable in many optical systems to be able to use prefabricated plastic or glass molded lens arrays in front of the laser array assembly, or even prefabricated fiberoptic waveguide ribbons or bundles butted against the assembly. In these cases, the individual lenslets of the lens arrays or the input apertures of the individual waveguides in the ribbons or bundles must be accurately aligned with the corresponding individual laser light emitters of the two-dimensional laser array assembly in order to properly transfer the output beam to where it is needed or to transform its image into a useful form, whether as a collimated beam or focussed spot. Even in cases where only simple lens systems are used, it is important that the spacing between submounts be precisely controllable, because such spacing affects the overall output beam shape of the array, and also because the uniformity or lack of uniformity of the spacing between laser bars affects the far field pattern of the beam.
In addition to the optical effects from a precise control of registration, there is also an important thermal effect. Because submounts are bonded together so as to be connected to an electrical power source in series, each submount is at a different electrical potential or voltage relative to ground. This means that a heat sink plate or block placed at the back end of the stacked submounts, and to which the submounts are bonded so as to remove heat from the submounts, must be electrically insulating to avoid shorting the submounts. If the back bonding is to be accomplished with high thermal conductance, then metal solders, rather than epoxy or other resins, are preferred. In order to use metal solders for bonding, the insulative heat sink plate or block must be premetalized in a registered pattern to avoid shorting, and the submount positions must register with the premetalized pattern. If such registration could be achieved, the low thermal resistance that would be provided by the better bond would allow the array assembly to be operated at much higher power levels.
However, mechanical tolerances of the support plate thickness, laser bar height and wire bonds make presently available submount structures difficult to align.
Accordingly, it is an object of the invention to provide a submount structure and corresponding bonding technique that enables a two-dimensional stacked diode laser array assembly to be constructed with more precise control over the separation and alignment of the submount structures that make up the assembly.